In the prior art it is known that the more efficient use of transmission line facilities may be achieved by the sampling of a plurality of digital data streams on relatively slow speed transmission lines by a multiplexer and the retransmission of the sampled data over a single transmission line at a relatively high speed transmission line. Many coding, timing, etc., schemes have been proposed for the serialization of such sampled data for transmission of the serialized data at the transmitting site while permitting reliable demultiplexing and reconstruction of the separate sampled data at the receiving site. See the article "Multiplexing and Concentration," D. R. Doll, Proceedings of the IEEE, Vol. 60, No. 11, November, 1972, pp. 1313-1321. The present invention is directed toward a method of optimizing the multiplexing and concentration of digital data for the high speed transmission thereof using a synchronous time-division multiplexing (STDM) scheme.